1. Field of the Invention
The present invention relates generally to a method of polishing C4 molybdenum (moly) masks used to pattern the placement of C4 balls on wafers and chips. The method removes moly peaks and projections from the surface of the C4 moly masks that cause PMD (polyimide mechanical damage) with resultant chip yield loss and cracked final hard passivation which presents a reliability risk.
2. Discussion of the Prior Art
Molybdenum (moly) masks are placed on all wafers which use the C4 process prior to the metalization process that places the C4 contact balls on the wafers. The moly mask is fabricated with a pattern of holes through the mask which determines the pattern of C4 contact balls to be fabricated on the wafers and chips. Unfortunately, the photolithographic process used to create the pattern of holes through the mask also roughens the topography of the surface of the mask, creating spikes and etch pits in the moly surface. In the prior art, moly masks are typically manually worked with a smoothing stone or are rolled prior to usage. The surfaces of the moly masks produced by prior art rolling or stoning methods are not as controlled and smooth as moly masks produced pursuant to the present invention with polishing operations as described herein.
Molybdenum is used as the mask material for wafers in the C4 process as its coefficient of thermal expansion closely matches the coefficient of thermal expansion of the wafers it is masking. Typically, a 225 mm diameter moly mask having a thickness of 125 150 um is clamped onto a wafer having a diameter of 200 mm. Moreover, the moly masks are typically reused on a plurality of different wafers.
One problem with prior art moly masks is that PMD (polymide mechanical damage) is created in the top polymide level or layer of the wafer when the moly mask is clamped to the wafer. The heat cycles which the wafer and the mask are exposed to cause the sharp peaks on the moly mask to compromise the poly and the underlaying metal lines on the wafer.